CODEWITHSUNDEEP
dns
user3332315
user3332315

Reputation: 141

Why are major DNS systems designed to write the subdomain before the root domain and not the reverse?

We write app.example.com instead of com.example.app in major DNS-based systems, including WWW, FTP, and email. What is the reason behind this design? Why not the reverse order?

Upvotes: 0

Views: 36

Answers (1)

Patrick Mevzek
Patrick Mevzek

Reputation: 12595

Hostnames existed before the DNS and even before TLDs.

Structures to the names have been added through RFC 921 "Domain Name System Implementation Schedule - Revised" (October 1984)

This document explains the change from simple names (no dot) to hierarchical ones, and this was needed because the Internet was growing at that time and a single flat list of names was not enough to describe every hosts.

Some excerpts:

The names are being changed from simple names, or globally unique strings, to structured names, where each component name is unique only with respect to the superior component name.

...

The elements (or components) of the structured names are separated with periods, and the elements are written from the most specific on the left to the most general on the right.

        For example:  USC-ISIF.ARPA

RFC 882 "DOMAIN NAMES - CONCEPTS and FACILITIES" (November 1983) just says it is a convention:

The domain name of a node or leaf is the path from the root of the tree to the node or leaf. By convention, the labels that compose a domain name are read left to right, from the most specific (lowest) to the least specific (highest).

A clue may come from RFC 1034 "DOMAIN NAMES - CONCEPTS AND FACILITIES" (November 1987) that repeats the above with some details:

The domain name of a node is the list of the labels on the path from the node to the root of the tree. By convention, the labels that compose a domain name are printed or read left to right, from the most specific (lowest, farthest from the root) to the least specific (highest, closest to the root).

RFCs (see RFC 1166) have the tradition to use the "MSB 0" bit numbering: it means that when you write down a byte, with 8 bits, you start with the most significant one, the bit with highest value (the encode encoding for the 128 decimal value). This was then extended with the concent of network byte order, where the most significant one is firsts.

I guess that the idea of starting with the most specific label of the name comes directly from this idea of the most specific bit first, which means starting with the label farthest from the root and hence finally having the root at the top right side and then reading a full name in a kind of right to left pattern.

Upvotes: 1

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